Some categories of water-cooled moderated nuclear fission reactor plants used for producing steam to generate electrical power, such as commercial ABWRs, utilize coolant water recirculation pumps having drive motors located in casings outside of the reactor pressure vessel (RPV). The drive motors are connected to pump impellers located within the RPV by means of drive shafts which pass through the wall of the RPV. Thus, in nuclear fission reactor plants of this type, a number of internal impeller pumps driven by external electrical or other motors are positioned in the lower or bottom portion of the RPV, each pump having a drive shaft extending through the wall of the RPV. This arrangement requires that a multiplicity of bore openings pass through the curved bottom portion of the RPV, which contains a large volume or head of recirculation coolant water under high temperature and high pressure for submerging the fuel core and transferring heat away from the fuel core and to equipment for converting steam into electricity.
Each bore opening in the RPV for passage of a pump drive shaft therethrough is formed by the bore of a short, hollow cylindrical stub which projects vertically upward from the vessel bottom. In one design, a hollow cylindrical housing supports the external drive motor casing and surrounds the drive shaft extending from the motor. The housing passes through the RPV wall with the drive shaft extending therefrom to the pump impeller for recirculation coolant water within the RPV. This hollow cylindrical housing extends concentrically within the bore of the stub and up into the RPV. The housing is secured, for example, by welding the upper annular end of the housing to the surrounding inside area of the main cylindrical stub bore.
Due to the routine mechanical movement of such driven impeller pumps, including high-speed rotation and inherent vibrations, which over a period of time can result in wear and metal stress and fatigue, components of this recirculation pump system are periodically serviced and may be replaced to ensure effective and safe operation of the nuclear reactor plant. Maintenance service of this kind may require replacement or repair welding of the hollow cylindrical housing which supports the drive motor casing and surrounds the pump drive shaft, and extends concentrically a distance into the stub bore.
Maintenance service of this recirculation system, including either replacement or repair welding of portions thereof, such as the upper annular end of the hollow cylindrical housing, the encircling portion of the hollow cylindrical stub or the stub-to-housing weldment therebetween, must be carried out under cumbersome and difficult circumstances. Namely, this maintenance procedure must be performed within the lower portion of the RPV while the vessel retains a substantial body of radioactively contaminated coolant water for covering and cooling the fissionable fuel core, which produces heat even during shutdown. Thus, the maintenance service, including repair welding of the upper annular end of the hollow cylindrical housing or stub or weldment therebetween, must be carried out beneath a high head of water which is radioactively contaminated. During the procedure, the coolant water cannot be permitted to escape through the pressure vessel bore opening that enables entry of the recirculation pump drive shaft up into the RPV and through which the recirculation pump assembly is installed. Service operations under such stringent circumstances require very complex procedures and special equipment and tools.